1. Field of the Invention
The present invention relates to an optical branching filter which mixes or branches different wavelengths of light to or from a single beam of light.
2. Description of the Related Art
An optical branching filter is a basic component of a wavelength dividing multiplexing optical communication system. For example, in a single line of optical fiber it is possible to use n different wavelengths .lambda..sub.1, .lambda..sub.2, . . . , .lambda..sub.n of light to transmit n times as much information as is possible using a single wavelength .lambda..sub.0 of light. Further, n-channel two-way simultaneous optical communication can be realized by allocating the different wavelengths of light for transmission and reception. This can be accomplished, for example, by allocating wavelengths .lambda..sub.1, .lambda..sub.3, . . . , .lambda..sub.2n-1 for transmission and wavelengths .lambda..sub.2, .lambda..sub.4, . . . , .lambda..sub.2n for reception.
In order to realize such an optical communication system, an optical mixing filter is required which mixes the different wavelengths of light and inputs the mixed wavelength lightbeam to a single line of optical fiber. The optical mixing filter is arranged at a sending side of the optical communication system. Also required is an optical branching filter which separates out the respective wavelengths of light from the mixed wavelength lightbeam, which is arranged at the receiving side of the system.
The optical mixing filter and optical branching filter in the prior art generally mix and branch the various wavelengths of light by employing a dielectric material filter. This filter is composed of a thin dielectric multilayer film laminated in a plurality of layers, such as SiO.sub.2 and TiO.sub.2. The filter acts as a bandpass filter, which allows the transmission of a particular wavelength of light, but reflects all other wavelengths of light.
The optical mixing filter can also be used as the optical branching filter, and visa versa, by inverting the incoming and outgoing directions of light to and from the dielectric material filter. These mixing and branching filters have the same structure, and these filters are both called optical branching filters. To fully realize this optical branching filter, further optical components, in addition to the dielectric material filter, are required. For example, these include a plurality of optical lenses, which cause the lightbeam output from the optical fiber to be effectively input into each dielectric material filter, and conversely causes the lightbeam from the dielectric material filter to be effectively input into the optical fiber, and a plurality of fixing mechanisms, which fix an end face of the optical fiber for proper focusing of the lenses.
However, the optical branching filter deals with the various wavelengths of light. A refractive index of a lens generally changes in accordance with the wavelengths of light, and therefore the focal point of a lens depends on the wavelength of light.
Therefore, since the fixing mechanisms require adjustment for the focal points for each wavelength, the manufacturing yield of optical branching filters deteriorates. This deterioration becomes even more distinct with an increase in the number of different wavelengths of light to be mixed or branched. This has become significant as various kinds of semiconductor lasers and light emitting diodes using a wide range of wavelengths from long to short have been developed. The variety of wavelengths dealt with by an optical branching filter can therefore be large, and thus the problem of adjustment for the numerous focal points is very serious.